Ecdysteroid Titer, Responsiveness of Prothoracic Gland to Prothoracicotropic Hormone (PTTH), and PTTH Release of the Recessive Trimolter Strain of Bombyx mori in Extra-ecdysed Larvae by JHA and 20E Application

Application of 20-hydroxyecdysone (20E) or juvenile hormone analogue (JHA) to the last (fourth) larval instar of a recessive trimolter Bombyx strain induced extra larval ecdysis. The ecdysteroid titer, the PTTH-secreting activity of the brain, and the ecdysteroid-secreting activity from the PG were compared among the untreated and 20E- and JHA-treated recessive trimolter. The ecdysteroid titer in the hemolymph and ecdysteroid secretory activity of the PG decreased in the mid-fourth larval instar in untreated control larvae, while those in 20E- and JHA-treated larvae did not decrease during the same stage. An application of JHA enhanced the PTTH-secreting activity of the brain. Results indicate that the recessive trimolter strain of Bombyx mori lost the responsiveness of the PG to PTTH in the early stage of the fourth larval instar, resulting in precocious metamorphosis. Results also indicate that the application of 20E induced extra larval ecdysis by maintaining substantial levels of ecdysteroid in the hemolymph. However, JHA is suggested to induce larval ecdysis through activation of the brain and maintenance of the responsiveness of the PG to PTTH.